As is known, in both metro and regional optical networks, there are many nodes that require adding and dropping wavelength division multiplexed (WDM) channels at various locations along an optical fiber link. In conventional WDM networks, channel add/drops at nodes are performed by reconfigurable optical add/drop multiplexers (ROADM).
A typical ROADM requires configurable optical filtering/routing functions—such as a wavelength selective switch—for optical signal add/drop—as well as optical amplifiers to compensate for insertion loss resulting from the optical filtering. The use of these amplifiers (active modules) adds to the cost of implementing ROADM nodes and requires a significant amount of electrical power supplied to nodes employing same along the fiber network—and in particular any optical fiber trunk line(s).
Recently, advances in coherent detection technologies has greatly improved the sensitivity of optical transponders. Such advances have enabled a technique—“LO selection”—in which an optical receiver can detect any channels among a transmitted WDM spectrum without using optical filtering. Additionally, such coherent technologies have also led to the development of multi-span fiber links with passive optical add/drop multiplexing (POADM) architectures. In a contemporary POADM architecture, a node will employ passive splitters at a branch terminal to add channels when empty spectral slots are available and allow transponders of downstream branch terminals to receive any channel conveyed by the fiber. Notwithstanding these developments however, such prior art architectures still employ optical amplifiers to compensate for loss due to any fiber characteristics and resulting from passive splitting and consequently still require significant amounts of electrical power at the nodes.